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Chapter-16(continued)
16.5 The Nature of Sound
Longitudinal Sound Waves
Sound in air is a longitudinal wave that is created by a vibrating object, such as a guitar string, the human vocal cords, or the diaphragm of a loudspeaker.
Sound cannot propagate in a vacuum
How do we hear?
Wave Picture
Graphic representations of a sound wave.
Air at equilibrium, in the absence of a sound wave;
Compressions and rarefactions that constitute a sound wave;
Transverse representation of the wave, showing amplitude (A) and wavelength (λ).
Name the x axis?
What are the choices for the y-axis?
Draw a transverse representation of a wave, name the axes, and show period.
The Frequency of a Sound Wave
Audible Range: 20 Hz ----- 20,000 Hz.
The Frequency of a Sound Wave
Audible Range: 20 Hz ----- 20,000 Hz.
Infrasonic waves: Sound waves with frequencies < 20 Hz.
Rhinoceroses use infrasonic frequencies as low as 5 Hz to call one another
The Frequency of a Sound Wave
Audible Range: 20 Hz ----- 20,000 Hz.
Infrasonic waves: Sound waves with frequencies < 20 Hz.
Rhinoceroses use infrasonic frequencies as low as 5 Hz to call one another
Ultrasonic waves: Sound waves with frequencies > 20,000 Hz.
Bats use ultrasonic frequencies up to 100 kHz for locating their food sources and navigating.
Objective and Subjective properties of sound
Objective properties can be measured, used in physics.
Subjective properties are subjective to the person, used in music.
Objective property Subjective quality
Frequency Pitch
Intensity Loudness
Waveform Tymbre or Quality
16.6 The Speed of Sound
Table 16.1 Speed of Sound (m/s)
Gases
Air (0 °C) 331
Air (20 °C) 343
Carbon dioxide (0 °C)
259
Oxygen (0 °C) 316
Helium (0 °C) 965
Liquids
Chloroform (20 °C)
1004
Ethyl alcohol (20 °C)
1162
Mercury (20 °C) 1450
Fresh water (20 °C)
1482
Seawater (20 °C) 1522
Solids
Copper 5010
Glass (Pyrex)
5640
Lead 1960
Steel 5960
16.7 Sound Intensity
The sound intensity I is defined as the sound power P that passes perpendicularly through a surface divided by the area A of that surface:
The unit of sound intensity is power per unit area, or W/m2.
Human Ear and Sensitivity
Audible frequency range: 20 Hz – 20,000 Hz
Audible intensity range: 10–12 W/m2 - 10 w/m2
10–12 W/m2 = Threshold of hearing
10 W/m2 = Threshold of pain
16.8 DecibelsThe decibel (dB) is a measurement unit used when comparing two sound intensities.
The intensity level (expressed in decibels) relative to the threshold of hearing, Io is defined as follows:
TABLE 16.2 Typical Sound Intensities and Intensity Levels Relative to the Threshold of Hearing
Intensity I (W/m2) Intensity Level (dB)
Threshold of hearing 1.0 × 10-12 0
Rustling leaves 1.0 × 10-11 10
Whisper 1.0 × 10-10 20
Normal conversation (1 meter) 3.2 × 10-6 65
Inside car in city traffic 1.0 × 10-4 80
Car without muffler 1.0 × 10-2 100
Live rock concert 1.0 120
Threshold of pain 10 130
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